This invention relates to electrophotographic processes and apparatus. In one of its more particular aspects this invention relates to an improved photoconductive coating and the process for producing such coating.
Electrophotographic processes for making copies of graphic originals using photoconductive media are well known. The basic process consists of applying a blanket electrostatic charge to a photoconductor in the dark and then exposing the charged photoconductor to a pattern of light and shadow created by directing electromagnetic radiation onto a graphic original. The light-struck areas of the photoconductor are thereby discharged leaving a latent electrostatic image corresponding to the original. A developed image is produced by applying an electroscopic powder to the latent electrostatic image and then fixing the image or transferring and fixing onto a suitable receiving medium such as plain paper. Variations of this process have also been utilized.
One such variation consists of the use of foraminated structures formed by applying a photoconductive layer to a conductive screen, grid or similar apertured substrate. Such structures function as ion modulators which can be made to selectively pass a stream of charged particles through the apertures in a pattern corresponding to a graphic original.
A simple two-layered screen construction formed by applying a photoconductive layer onto an apertured metallic substrate is disclosed in U.S. Pat. No. 3,220,324 to Christopher Snelling. This screen can be used to apply an electrostatic charge pattern to a dielectric target. The pattern applied corresponds to the pattern of light and shadow created by electromagnetic radiation of a graphic original.
Many different types of both organic and inorganic photoconductors have been used in electrophotographic processes. However, because of its desirable electrical properties, selenium is the most widely used photoconductor.
Selenium is generally applied to surfaces to form a photoconductive layer thereon by techniques of evaporation or sputtering. In evaporation techniques selenium is heated and the vapors are condensed upon the desired substrate. Evaporation is generally conducted under a vacuum of 10.sup.-5 torr or better in order to minimize the amount of gaseous impurities present which adversely affect the photoconductive properties of the selenium applied.
In sputtering a selenium cathode is caused to sputter selenium atoms upon a substrate by application of a high radio frequency (r.f.) electrical potential between the cathode and an anode, between which the substrate is placed. Sputtering is also accomplished under a vacuum in the presence of an inert gas.
In general, selenium layers having thicknesses in the range of about from 10 microns to 200 microns are produced by means of the known evaporation and sputtering methods. However, with these relatively large thicknesses, it has sometimes been found that the current carrier range is inadequate resulting in the trapping of electrons or holes and the consequent failure to meet performance expectations.
It has also been difficult in the past to obtain uniform selenium coatings especially when using foraminated substrates.